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Font Strings and Measurement

Every preparation API takes a font string. The active measurement backend decides how to interpret that string, but the first-party backends share one parser from Pretext.Contracts.

Accepted format in the first-party backends

The parser looks for:

  • a size in px
  • an optional weight before the size
  • optional italic or oblique
  • a comma-separated family list after the size

Examples:

"16px Inter"
"700 18px \"IBM Plex Sans\""
"italic 16px Georgia"
"600 15px system-ui"
"16px Menlo, monospace"

Supported style tokens

Token Effect
bold weight 700
numeric weight such as 500 or 700 mapped to backend-specific weight values
italic or oblique italic slant

Family fallback rules

If the primary family is a generic CSS family, the first-party backends map it to a concrete platform fallback:

Input family Effective family
sans-serif, system-ui backend-specific sans-serif fallback
serif backend-specific serif fallback
monospace, ui-monospace backend-specific monospace fallback

If the parser cannot find a valid size, it falls back to 16px with the backend's default sans-serif family.

What is not parsed

  • %, em, rem, and other non-pixel units
  • advanced font-variation syntax
  • OpenType feature strings
  • line height embedded in CSS shorthand

If a /line-height segment appears after the size, it is ignored by the parser. You still pass line height explicitly to layout methods.

Measurement model

Pretext stores:

  • width of each prepared segment
  • discretionary hyphen width
  • tab stop advance, based on eight space widths
  • per-grapheme break widths for breakable runs

When you use one of the first-party backends, widths come from that backend's native or renderer-specific text stack:

  • Pretext.DirectWrite on Windows
  • Pretext.FreeType on Linux
  • Pretext.CoreText on macOS
  • Pretext.SkiaSharp as the portable fallback

Because line height is caller-supplied, you stay in control of:

  • baseline policy
  • paragraph spacing
  • host-specific font metrics
  • visual density choices

Practical advice

  • Use the same font string for both preparation and rendering.
  • Keep the font string stable while only width changes.
  • Pick a line height from your host UI system rather than assuming font size * constant.